Wavelength Dependence of Micro-EHL Pressure Rippling with Random Surface-Roughness Profiles

This paper studies line-contact elastohydrodynamic lubrication (EHL) between two rough surfaces of random roughness profiles. A transient micro-EHL model is used to simulate the problems. Numerical results are obtained for a range of practical operating conditions and for relatively small specific film thicknesses (i.e., the A-values). The study reveals the wavelength dependence of micro-EHL pressure rippling. For the same A-value, the size of the pressure rippling is shown to be significantly smaller than those predicted earlier using more idealized system models. Furthermore, the pressure distribution is essentially unaffected by the short-wavelength components in the random surface-roughness profiles, except under near rolling conditions. It is shown that lubricant non-Newtonian shear-thinning is the mechanism that suppresses the short-wavelength pressure ripples. With a Newtonian lubricant, sharp pressure rippling is generated by the small-scale surface roughness textures. Since EHL lubricants exhibit significant shear-thinning and since the surfaces possess random roughness profiles, the results reported in this paper reflect a main feature that may prevail in realistic EHL conjunctions. Future work will establish a theoretical basis for the numerical analysis conducted in this research.     

Investigating the Effect of Surface Finish on Mixed EHL in Rolling and Rolling-Sliding Contacts

Surface finish may significantly affect the lubrication performance of a tribological interface through the influence of topography on micro/nanoscale fluid flows around localized contacts at surface asperities. This paper aims to study the mixed lubrication performance of a group of engineered surfaces, including turned, isotropically finished, ground, and dimpled surfaces, under different operation conditions by means of a deterministic mixed elastohydrodynamic lubrication (EHL) model. The honed surface was used to mate with other surfaces. The results indicate that a longitudinal contact ellipse favors longitudinally oriented mating surface roughness and that a transverse contact ellipse, as well as a line contact, prefers a transversely orientated mating surface roughness for lubrication enhancement.     

A New Method for Rough Surface Profile Simulation Based on Peak–Valley Mapping

Starting from random process theory and taking peaks and valleys as reconstruction carriers to replace surface height distribution and autocorrelation function (ACF), a new method for rough surface simulation is proposed based on the peak–valley mapping principle. The proposed method is used to simulate rough surfaces with different correlation lengths. The simulated roughness parameters and ACF are examined. Comparisons of simulation precision and simulation efficiency between the proposed method and fast Fourier transform (FFT) are provided. Finally, the applications of the method are discussed briefly.     

Investigation of Oil Additives for Boundary Lubrication of Railroad Journal Bearings

The boundary friction properties of the lubricated system of steel sliding on lead-base babbitt were investigated as a phase of the Association of American Railroads' sponsored project on freight car hot boxes. A friction pendulum was developed for determining the coefficients of boundary friction as a function of temperature with the specified metals lubricated with various lube oil additive blends.

A group of general relationships dealing with additives for the steel-babbitt system was developed. It is concluded that the extreme pressure agents containing sulfur or chlorine are generally unsuited for railroad journal bearing use. Lubricity additives which function by simple physical adsorption were found to be ineffective at moderate or high temperatures existing in railroad operations. Those lubricity additives which chemisorb to metal surfaces and form a close-packed layer were found to be effective antifriction agents.     

On Mechanisms of Fatigue Life Enhancement by Surface Dents in Heavily Loaded Rolling Line Contacts

This paper studies mechanisms of surface dents in enhancing the fatigue life of rolling bearings previously reported in Akamatsu et al. (1). First, transient micro-EHL analyses of heavily loaded contacts between rough surfaces with multiple dents are conducted under near rolling conditions. Contacts with various dent dimensions, dent arrangements under different loading and kinematic conditions are investigated. Results show that surface dents generate no favorable micro-EHL effects to enhance the contact fatigue life. Subsequent analyses, in conjunction with other published studies, suggest that the fatigue life enhancement likely comes from the reduced local traction at asperity contacts through the “oil pots” effects of the dents. The effects of the surface dents on contact fatigue life may depend on the lubrication regime in which the contact is operating being favorable in poor lubrication conditions but adverse in well-lubricated contacts. Since rolling bearings are usually designed to operate in a healthy regime of lubrication, fatigue life enhancement by artificially introducing dents on bearing surfaces may not extend to field applications.     

On the Modeling of Elastic Contact between Rough Surfaces

The contact force and the real contact area between rough surfaces are important in the prediction of friction, wear, adhesion, and electrical and thermal contact resistance. Over the last four decades various mathematical models have been developed. Built on very different assumptions and underlying mathematical frameworks, model agreement or effectiveness has never been thoroughly investigated. This work uses several measured profiles of real surfaces having vastly different roughness characteristics to predict contact areas and forces from various elastic contact models and contrast them to a deterministic fast Fourier transform (FFT)-based contact model. The latter is considered “exact” because surfaces are analyzed as they are measured, accounting for all peaks and valleys without compromise. Though measurement uncertainties and resolution issues prevail, the same surfaces are kept constant (i.e., are identical) for all models considered. Nonetheless, the effect of the data resolution of measured surface profiles will be investigated as well. An exact closed-form solution is offered for the widely used Greenwood and Williamson (GW) model (Greenwood and Williamson, Proceedings of the Royal Society of London A, vol. 295, pp. 300–319), along with an alternative definition of the plasticity index that is based on a multiscale approach. The results reveal that several of the theoretical models show good quantitative and qualitative agreement among themselves, but though most models produce a nominally linear relationship between the real contact area and load, the deterministic model suggests otherwise in some cases. Regardless, all of the said models reduce the complicated surface profiles to only a few key parameters and it is therefore unrealistic to expect them to make precise predictions for all cases.     

On Prediction of Wear Coefficients in Sliding Wear

The wear rates and wear coefficients of metals are analytically predicted based on the delamination theory of wear when the wear rates are controlled by the subsurface crack propagation rate. The wear rate and the wear coefficient are predicted to be directly proportional to the depth of crack location and the crack growth rate. The numerical values of wear coefficients are obtained through finite element analysis of crack propagation in elastoplastic solids. The agreement between typical experimental results and theoretical prediction is excellent.     

Wear Behavior of Thin Film Heads for VCR in Sliding with Metal Evaporated Tapes

In order to apply thin film heads to digital video cassette recorders, wear behavior, machinability of head materials, and level difference of the heads were evaluated. The tests, performed with metal evaporated tapes, were divided into two stages. First, wear rate and machinability of individual materials were evaluated. It was found that adhesive wear was dominant for every material. Wear damage was especially-severe for metal magnetic films. Machinability was satisfactory for most bulk materials except for those with a hardness of more than 1000 kgf/mm², ZrO₂ ceramics and devitalized glass. Secondly, wear damage and level difference were evaluated using model heads. After the test, a number of flows were formed on sliding surfaces, mainly on the film surfaces, indicating that abrasive wear occurred. With bulk material of larger wear rate, the level difference reached the final value faster and its deviation was smaller. With Co-Zr-Nb magnetic metal and an Al₂O₃ protective layer, the level difference was less than 20 nm using CaTiO₃ or ZrO₂-Ta₂O₅, bulk substrates.     

A Simplified Model to Study EHL Film Collapse During Rapid Halting Motion

A theoretical model for describing the EHL film thickness during rapid deceleration is presented. The theory is based on the pioneer work of Ertel (1939) and Grubin (1949), who gave the first analytical solution for the elastohydrodynamic lubrication of a line contact under stationary operating conditions. An extension is made here for rapid halting motion. The proposed model is well adapted when the halting period is small in comparison to the transit time (i.e. 2b/u, ratio between the contact width and the rolling speed). This work completes the model of Glovnea and Spikes (2001b), appropriate for slow halting motion but which suffers from experimental fitting, and the model of Chang (2000) that is more suitable for speed or load oscillations at a wavelength close to the transit time.

This behavior implies that stop-start, reciprocating or rapidly halting machine components may be able to maintain a separating film for longer than would be expected based on steady-state EHL theory. An application to a ball bearing arrangement in a space mechanism is finally made in order to assess the model capabilities.     

The Role of Slider/Disk Roughness in 1 Tb/in.2 Magnetic Recording

To achieve 1 Tb/in.² magnetic recording areal density, the head/disk spacing, or the flying height of the slider, has become so small that both the disk surface roughness and the slider air-bearing surface roughness need to be considered. In this region, the intermolecular force and the contact force become more significant due to the roughness of the two surfaces. This article targets two points: 1) slider/disk roughness effects on intermolecular force and 2) slider/disk roughness requirement for 1 Tb/in.² areal density. A probability model is built to simulate the intermolecular force and the contact force, and these two forces are introduced into the modified compressible Reynolds equation governing the air-bearing pressure of the slider. The equation is solved by the finite volume method based on an unstructured triangle-based mesh. The simulation results show that in 1 Tb/in.² areal density magnetic recording the effects of slider/disk roughness on the intermolecular force are negligible. Smaller R _a values will have fewer effects on flying performance.     

A Study of Dynamically Loaded Finite Journal Bearings in Mixed Lubrication Using a Transient Thermohydrodynamic Analysis

A transient analysis of dynamically loaded finite journal bearings in mixed lubrication is made by solving the modified generalized Reynolds equation, 2-D energy equation in the oil, and heat conduction equation in the journal simultaneously including mass conserving cavitation. ISOADI (isothermal shaft and adiabatic bushing inner surface) thermal boundary condition is used. Journal temperature is treated as quasi-steady over one loading cycle. Two kinds of contact model are used. Numerical solutions using the finite difference method are presented. Results shows that the bearing behavior is closely tied to the roughness texture and topography, asperity contact load, bearing geometry, and operating conditions.     

The Effect of Scale-Dependent Hardness on Elasto-Plastic Asperity Contact between Rough Surfaces

Statistical methods are used to model elasto-plastic contact between two rough surfaces using a recent finite element model of elasto-plastic hemispherical contact and also recent advances in strain gradient modeling. The elasto-plastic hemispherical contact model used to model individual asperities accounts for a varying hardness effect due to deformation of the contact geometry that has been documented by other works. The strain gradient model accounts for changes in hardness due to scaling effects. The contact between surfaces with hypothetical material and surface properties, such as the elastic modulus, yield strength, and roughness are modeled. A model is also constructed to consider a variable asperity contact radius to evaluate if the strain gradient model will affect it differently. The models produce predictions for contact area, contact force, and surface separation. The strain gradient effects decrease the real area of contact and increase the average contact load in comparison to the model without these effects. The strain gradient model seems to have a larger influence on the predictions of contact load and area than does considering a variable asperity contact radius for the cases considered in this work.     

Cam and Tappet Lubrication. IV–Radioactive Study of Sulfur in the EP Film

A radioactive tracer, sulfur-35, synthesized into zinc dialkyl dithiophosphate molecules, was used to study the EP film formed on cast iron cams and tappets run in motor oils containing this additive. The sulfur content of static films increased with immersion time and temperature, and the presence of phosphate-coated metal surfaces. The bound sulfur of films formed during dynamic tests increased with running time, load, and with the use of phosphate-coated surfaces. These conditions also influenced the ratio of Zn:P:S contained in both static and dynamic films. Zinc, and particularly phosphorus, in the films increased more rapidly than the sulfur with increased temperature and/or pressure. The dynamic films are not easily worn off by running in nonadditive oil. The mechanism of action of zinc dithiophosphates appears to be related to chemical reactions of additive decomposition products with the metal surfaces to form tightly-bound solid films which reduce damage under extreme pressure conditions. Radioactive counting and X-ray spectroscopy were used to obtain the amount of sulfur and zinc on tappets. Densitometer traces of the autoradiographs were utilized to determine the distribution of the sulfur on cam and tappet surfaces.     

A Mixed Lubrication Model for Computer Simulation of Extrusion Processes

A mixed lubrication/friction model for extrusion process is developed in the present research. The model combines a rigid-plasticity finite element code to simulate the interface condition between the tooling and workpiece in the extrusion operation. The influence of surface roughness on lubricant flow is treated by using the average Reynolds equation. The active lubrication regime and appropriate friction factor were determined from the current local values of interface variables such as mean lubricant film thickness and workpiece and tooling roughness, in addition to the more traditional external variables such as interface pressure, node sliding velocity and strain rate of the workpiece. Numerical results using the coupled code include friction stress and normal pressure under different lubrication conditions are compared with experimental investigation. The discrepancy is very small and the proposed model proved to be very efficient in predicting interface friction condition in the extrusion processes.     

A Numerical Model for the Dry Sliding Contact of Layered Elastic Bodies with Rough Surfaces

A numerical model for the two-dimensional dry sliding contact of two elastic bodies with real rough surfaces has been developed, where an elastic body contacts with a multi-layer surface under both normal and tangential forces. The model uses surface profile data directly recorded with a stylus measuring instrument and it is suitable for use on a microcomputer. Green's function for a unit normal load and a unit tangential load for the generalized plane strain problem are derived. Verification of the accuracy of the model by reproduction of test case results is presented. Contact pressure distribution for layers of varying coefficient of friction, thickness and elastic modulus is analyzed.     

Calculated Performance of Non-Newtonian Lubricants in Finite Width Journal Bearings

A numerical procedure has been developed for the calculation of the performance of non-Newtonian, polymer-thickened lubricants in finite width journal bearings. Such oils were found to act as if they had averaged “anisotropic viscosities,” i.e., different viscosities in the circumferential and side leakage directions, even though the viscosity was taken to have one definite value, a function of the resultant shear stress, at each point in the oil film. Overall, polymer oils carried less load at a given eccentricity, gave less friction and a flatter pressure distribution than mineral oils of the same low shear viscosities. By analogy with the previously calculated infinite width case, which gave similar results, it is expected that the flatter viscosity temperature slope of the polymer oils will compensate for their apparent viscosity decrease. The program has also been adapted to “natural” boundary conditions, which improve upon the delineation of the cavitation region on the inlet side of the bearing.     

The role of micro-cavitation on EHL: A study using a multiscale mass conserving approach

The role of micro-cavitation in Elastohydrodynamic Lubrication is numerically investigated using a multiscale approach whereby both the small scale topographical features and the micro-cavitation of the lubricant due to the features are resolved. Micro-cavitation and the fluid׳s shear-thinning property are modelled at the small scale of topological feature. The effects of topographical features on the film thickness of the line contact bearings and friction coefficient are presented with a focus on the role of micro-cavitation. This highlights how a mass conserving small scale model can be used to model both micro-cavitation and cavitation occurring at the bearing scale, and how topological features can be designed to reduce friction while maintaining bearing load.     

Friction and Wear of Refractory Compounds

The sliding characteristics of the borides, carbides, nitrides, and silicides and oxides of several metals were investigated in air at temperatures up to 2000 F. Tests were of the crossed-cylinder type and friction and wear effects measured under repetitive sliding conditions.

Correlation of the friction and wear characteristics with known or predicted solid solubility was only fair. High hardness coupled with low fracture strength and excessive brittleness of these materials produced fracturing of asperities and abrasive wear before a true measure of the adhesion could be obtained. Oxidation products were found to provide some lubrication at elevated temperatures, particularly the formation of B₂O₃ on boron carbide. However, none was effective at room temperature.     

高速铣削表面粗糙度建模与预报

在五轴高速加工中心上,采用多元回归正交试验法对45#中碳钢和P20模具钢进行了高速铣削试验。基于概率统计和回归分析原理,建立了表面粗糙度预测模型,并对回归方程和回归系数分别进行了显著性检验。通过分析正交试验直观图,研究了铣削用量对表面粗糙度的影响。为合理选择铣削参数提供了可靠的依据。     

Calculation of a Safety Margin for Hydrogenerator Thrust Bearings

The purpose of this paper is to present the two-dimensional linear stability analysis considering the fluid flow in both full film and cavitation regions for a plain cylindrical journal bearing. The Lund's infinitesimal perturbation procedure is applied to Elrod's universal equation for evaluation of unsteady pressure gradients. Based on JFO theory, the pressure distribution, film rupture, and reformation boundaries can be obtained using Elrod's universal equation, for a given operating position of the journal. In this work, it is assumed that for infinitesimal perturbation of a journal about the equilibrium position, the film rupture and film reformation boundaries are the same as those obtained for steady state. However, the unsteady pressure gradients in the full film region are evaluated taking into consideration the perturbed flow parameters in the cavitation region, i.e., at both rupture and reformation boundaries. The linearized stiffness and damping coefficients, whirl frequency ratio, and threshold speed for various values of eccentricity and L/D ratios are obtained for a plain cylindrical journal bearing with an axial groove along the load line. Measured data of dynamic coefficients for a 120° partial arc bearing are chosen for comparison with this work. Results show good agreement between the theoretical and experimental results.